Rat liver regeneration provides an opportunity to analyze the control of growth in vivo. Normally quiescent hepatocytes undergo a rapid change after partial hepatectomy that leads to proliferation. Cross circulation studies show that the signal is humoral and other data indicate that insulin and glucagon, acting via a cAMP-mediated process, are involved. Epidermal growth factor (EGF), a known hepatic mitogen, has also been implicated but there is little evidence to substantiate its relevance to regeneration. Aspects of the intracellular response have been documented including a temporal relationship between DNA synthesis and localized increases in guanylate cyclase activity. We have recently made new observations that form the basis of the proposed studies: 1) primary cultures of rat hepatocytes provide an excellent model in which to study the hormonal control of guanylate cyclase activity under defined conditions. 2) EGF receptor number decreases after partial hepatectomy. Hepatocyte cultures will be used to study the factors that alter the activity and subcellular distribution of guanylate cyclase after partial hepatectomy. We will characterize the mechanisms by which these alterations occur. The hepatocytes will be used to determine the effect of alteration in guanylate cyclase activity on cGMP compartmentalization by studying basal cGMP tissue concentration and the pattern of cGMP immunofluorescence in control and induced cultures. The acute effect of stimuli including EGF on cGMP and cGMP immunofluorescence will also be determined in control and induced cultures. Combined in vivo and in vitro studies are planned to demonstrate that EGF is a hormone relevant to regeneration. The plasma concentration of EGF at various times during regeneration will be determined as will the plasma half-life of EGF. We will attempt to identify the factor from plasma of rats undergoing liver regeneration that "down regulates" EGF receptor number using an assay based on the response of cultured hepatocytes.